It is often desirable to accurately monitor or measure the mass flow of a fluid flowing through a duct, and various types of instruments have been developed for such purpose. Mass flow measurements have taken on particular significance with the rapidly increasing concern for optimizing the clean, efficient functioning of internal combustion engines, especially those which find application in automobiles and the like. Through an accurate determination and/or control of the mass flow of air to an engine, a more efficient proportioning and mixing of fuel with the air for subsequent combustion is possible. In such automotive applications, it is desirable that a suitable mass flow sensor and meter be accurate and particularly durable yet relatively inexpensively mass produced.
Devices for mass flow measurement of gases and liquids have been developed and extensively used in the chemical process industry. Many of these proven techniques are not applicable to measuring mass flow of air in the internal combustion engine. The primary reason for this is that many of the devices require auxiliary sensors (e.g., vortex shedding meters, rotameters) to provide measurements from which mass flow can be calculated.
In one automotive application, an ultrasonic transducer senses air velocity which is then used in conjunction with a pressure sensor to compute mass flow. In another such application, a hot wire sensor has been considered for directly measuring mass flow.
A mass flowmeter which may meet some of the foregoing criteria is described in an article entitled "Linear Momentum Mass Flowmeter Design and Performance" by Rusnak, Wuori and Minell appearing at page 871 in FLOW, ITS MEASUREMENT AND CONTROL IN SCIENCE AND INDUSTRY, Instrument Society of America, copyright 1974. In the flowmeter described, the sensor included a dualsyn transducer, a torsional spring, and a drag body which was introduced to the flow stream. The dualsyn transducer is an inductive-type device with both torque and rate windings on the same stack. The torque winding is driven to cause oscillation of a rotor shaft having the drag body mounted thereon. The mass flow of air or other fluid past the drag body serves to damp the oscillations of the rotor, which damped oscillations are sensed and fed back via the rate windings. Circuitry intermediate the feedback rate winding and torque, or driving, winding operates to maintain the amplitude of the systems oscillations at a constant value. Thus, as the system damping increases due to an increase in the mass flow of the fluid, the loop gain is automatically increased to sustain the oscillation. It will be recognized that the rotary mechanism of such a sensor may be unduly complex and delicate for utilization in various automotive applications.
U.S. Pat. No. 3,368,219 issued Feb. 6, 1968 to C. E. Quinn for Liquid Level Indicator and U.S. Pat. No. 3,625,058 issued Dec. 7, 1971 to G. H. Endress et al for Apparatus for Determining the Filling Level of a Container, each describe devices which utilize electrically driven and sensed vibrating elements for determining the level of liquid or other fluid in a container. Magnetic or piezoelectric means are employed to drive the vibratory element at its natural frequency and corresponding electromagnetic or piezoelectric pickup means sense the existence and amplitude of such vibrations and apply them in a feedback manner to control the drive means. Generally speaking, other circuitry detects whether or not the predetermined amplitude of vibrator oscillations is occurring and utilizes such determination as an indication that the contained fluid has or has not reached a certain level. These devices do not, however, provide an indication of fluid mass flow.
Yet another patent, U.S. Pat. No. 4,193,010 issued Mar. 11, 1980 to H. W. Kompanek for Sensor Device Using Piezoelectric Coating Subjected to Bending discloses a bendable sensing element having a piezoelectric coating thereon and a pair of electrodes affixed thereto for respectively vibrating and sensing the vibrations of the bendable element. As in the instance of the sensors described in the two aforementioned patents, this sensor is also connected to circuitry for detecting a characteristic vibrational frequency or amplitude of the sensor to indicate contact or noncontact with various materials so as to determine the presence thereof.
A relatively simple and inexpensive yet durable device is needed which can quickly and accurately determine the mass flow of a fluid such as air through a duct, particularly in automotive applications.